Dúvida 21.2.3

[Renato]

Então, estou numa parte do HTDP onde passamos funções como parametros
por exemplo:

;; Exercise 21.2.2. Use map to define the following functions:

;; 1. convert-euro, which converts a list of U.S. dollar amounts into
;; a list of euro amounts based on an exchange rate of 1.22 euro for
each dollar

;; 2. convertFC, which converts a list of Fahrenheit measurements to a
list of Celsius measurements;

;; (define (map f lon)
;; (cond
;; [(empty? lon) empty]
;; [else (cons (f (first lon))
;; (map f (rest lon)))]))

;; convert-euro-from-map : (listof numbers) -> (listof numbers)
(define (convert-euro-from-map lon)
(map convert-euro lon))

;; convert-euro : number -> number
(define (convert-euro number)
(* 1.22 number))

;; convert-fahrenheit-from-map : (listof numbers) -> (listof numbers)
(define (convert-fahrenheit-from-map lon)
(map convert-euro lon))

;; convert-euro : number -> number
(define (convert-fahrenheit number)
(* 1.8 number))

Como da pra ver, utilizo "f" para passar uma função como parametro
dentro da construção de uma lista:

;; (define (map f lon)
;; (cond
;; [(empty? lon) empty]
;; [else (cons (f (first lon)) <--------
;; (map f (rest lon)))])) <--------

;; convert-euro-from-map : (listof numbers) -> (listof numbers)
(define (convert-euro-from-map lon)
(map convert-euro lon))

Agora no exercicio 21.2.3, tenho que passar como parametro uma função
que vai fazer a validação de uma condicional.

Exercise 21.2.3. Here is the version of filter that DrScheme
provides:

;; filter : (X -> boolean) (listof X) -> (listof X)
;; to construct a list of X from all those items on alon
;; for which predicate? holds
(define (filter predicate? alon)
(cond
[(empty? alon) empty]
[else (cond
[(predicate? (first alon)) <---------------
(cons (first alon) (filter predicate? (rest alon)))]
<---------------
[else (filter predicate? (rest alon))])])) <------------

Use filter to define the following functions:

1. eliminate-exp, which consumes a number, ua, and a list of toy
structures (containing name and price) and produces a list of all
those descriptions whose price is below ua;

2. recall, which consumes the name of a toy, called ty, and a list
of names, called lon, and produces a list of names that contains all
components of lon with the exception of ty;

3. selection, which consumes two lists of names and selects all
those from the second one that are also on the first.

Simplesmente não to conseguindo pensar criativamente com relação a
isso, eu ja fiz umas ideias rodarem, mas elas nao fazem o que
realmente é pedido.

I need air support, can you engage?

Valeu raça!
Abraço!